Staffordshire, United Kingdom: Increased cannabis use by the public has not been followed by a proportional rise in diagnoses of schizophrenia or psychosis, according to the findings of a forthcoming study to be published in the journal Schizophrenia Research.

Investigators at the Keele University Medical School in Britain compared trends in marijuana use and incidences of schizophrenia in the United Kingdom from 1996 to 2005. Researchers reported that the "incidence and prevalence of schizophrenia and psychoses were either stable or declining" during this period, even the use of cannabis among the general population was rising.

"[T]he expected rise in diagnoses of schizophrenia and psychoses did not occur over a 10 year period," authors concluded. "This study does not therefore support the specific causal link between cannabis use and incidence of psychotic disorders. ... This concurs with other reports indicating that increases in population cannabis use have not been followed by increases in psychotic incidence."

The results of a separate clinical trial published earlier this month reported that the recreational use of cannabis does not stimulate the production of dopamine in a manner that is consistent with the development of schizophrenia.

Last year, British lawmakers reclassified cannabis possession from a verbal warning to a criminal offense punishable by up to five years in jail. Lawmakers called for the increased penalties in large part as a response to allegations that marijuana use was linked to rising incidences of mental illness.

For more information, please contact Paul Armentano, NORML Deputy Director, at: paul@norml.org. For additional information on cannabis use and mental illness, please see the NORML white paper, "Cannabis, Mental Health, and Context," available online at: http://norml.org/index.cfm?Group_ID=6798. Full text of the study, "Assessing the impact of cannabis use trends in diagnosed schizophrenia in the United Kingdom from 1996 to 2005," will appear in Schizophrenia Research.